They often do loosen over time, as parts wear and all that. They rarely tighten themselves except when a valve seat wears through and the valve rests against soft metal; then that erodes real fast, and the valve moves upward in the head, and it can get to the point where the valve train holds it open.
I don't even know if a 2.8 has adjustable rockers. If it does, you could try adjusting them. A rolled cam lobe also makes what sounds like a tick, although if you could slow it down, it would probably be a squeak.
I don't think you have to pull the intake to get the valve covers off but i could be wrong. I don't know anything at all about the details of the 6-cylinder motors.
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